The present invention relates generally to variable attenuation signal acquisition probes and more particularly to variable attenuation signal acquisition probing systems using electro-optical cavities that are incorporated into voltage measurement systems.
Electro-optic material is a class of inorganic and organic crystals where the index of refraction of the material changes in response to electromagnetic energy applied to the material. Such material may be used in the production of optical devices, such as optical switches, optical limiters, optical modulators and the like. In it simplest form, an optical signal, such as the output of a laser or the like, is launched into the electro-optic material having length and widths in the millimeter range and thicknesses in the tenths of millimeter range. The diameter of the optical path of the optical signal within the electro-optic material generally ranges from ten to a few hundreds microns across. Electrodes are formed on opposing surfaces of the electro-optic material that are parallel to the optical path of the signal passing through the electro-optic material. An electrical signal is applied to the electrodes which varies the index of refraction of the electro-optic material as a function of the variations of the electrical signal. The variations of the index of refraction of the electro-optic material alters the optical signal propagating through the electro-optic material.
Optically reflective material may be disposed on opposing sides of the electro-optic material to form an optical cavity. A Fabry-Perot etalon is an example of such an optical cavity. The reflectivity of the optically reflective material on the opposing sides of the electro-optic material is defined by the particular application of the optical cavity. The optical signal passes through at least one of the optically reflective materials and into the electro-optic material. Electrodes are formed on opposing surfaces of the electro-optic material that are parallel to the optical path of the optical signal. An electrical signal applied to the electrodes varies the index of refraction of the electro-optic material as a function of the variations in the electrical signal.
The strength of the electric field distribution within the electro-optic material is a function of the distance between the opposing electrodes and the amplitude of the applied electrical signal. The strength of the electric field is the inverse of the distance separation of the electrodes. As the distance between the electrodes decreases, the strength of the electric field between them increases. As the distance decreases, the magnitude of the electrical signal can decrease to generate the same amount of change in the index of refraction.
Currently, the minimum overall dimensions of the electro-optic material used in optical devices and cavities is limited by the practical size at which the material can be handled resulting in electrodes that are positioned at a substantial distance from the optical path of the optical signal. This results in optical devices having low sensitivity to the applied electrical signal.
There is an increasing need in the electronics industry for measurement test equipment, such as oscilloscopes, logic analyzers and the like, to measure electrical signals in the gigahertz range. Correspondingly, there is a need for measurement instrument signal acquisition probes that have the signal bandwidth to acquire such high frequency signals. Generally gigahertz bandwidth signal acquisition probes have active circuitry in the probing head of the probe that receives the electrical signal via a metal probing tip extending from the end of the probing head. Extensive design work is required to minimize probe tip inductance and capacitance that affect the overall bandwidth of the probe. In addition, the dielectric constant of the probe head material also needs to be minimized for gigahertz differential signal acquisition probes. A further complication for gigahertz signal acquisition probe designs is the signal loss through the coaxial cable that couples the probing head to the measurement instrument. Additionally, some measurement probes provide varying amounts of attenuation to the measured electrical signal. Generally, the measurement probe provides a set amount of attenuation to the measured signal, such as 1×, 10×100×. The attenuation is added into the measurement probe circuit by the inclusion of a variable attenuator that is changes using manual or electronic switching.
U.S. Pat. No. 5,808,473, titled “Electric Signal Measurement Apparatus Using Electro-Optic Sampling by One Point Contact” describes an electro-optic sampling high-impedance probe exploiting the Pockels effect to rotate the polarization state of a light beam. The Pockels effect changes the birefringence of an electro-optic crystal by an amount that is proportional to an electric field inside the crystal. With the proper application of electrodes to the crystal surface, and their connection to conductive probing tips, the polarization rotation can be made to respond to a voltage on a device under test (DUT). The electro-optic sampling high-impedance probe receives polarization maintained laser pulses via a single mode polarization maintaining fiber. The laser pulses are coupled through bulk optic devices onto an electro-optic element having a reflective film on one end. A metal pin in the end of the signal probe head abuts the reflective film on the electro-optic element. The metal pin couples an electrical signal from a device under test to the electro-optic element which alters the birefringence of the electro-optic element in response to the electrical field of the signal causing the polarization state of the laser beam to change. The laser beam having the changed polarization state is reflected by the reflecting film and coupled through polarization beam splitters which convert the S and P polarized beams into an intensity change. The S and P polarized beams are coupled through respective condensing lenses onto respective slow germanium photodetectors that convert the optical beams into electrical signals. The electrical signals are coupled to a measurement instrument and detected by a differential amplifier.
U.S. Pat. No. 6,166,845 describes a modification to the above described electro-optic sampling high-impedance probe. Instead of coupling laser pulses via a single mode polarization maintaining fiber to the probe, a laser diode is incorporated into the probe itself. The laser diode generates a pulsed laser output in response to an input pulse chain from the measurement instrument. The probe contains the bulk optic devices, electro-optic element and photodetectors as previously described. The metal pin couples the electrical signal from a device under test to the electro-optic element which alters the birefringence of the electro-optic element in response to the electrical field of the signal causing the polarization state of the laser beam to change. The S and P polarized beams are coupled through the beam splitters and the condensing lenses onto the photodetectors. The photodetectors convert the intensity beams into electrical signals and couple the electrical signals to the measurement instrument.
A drawback to this type of probe is the size of the probing head due to the number of optical elements contained therein. Further, voltage and signal lines are required to couple the voltage power to the laser diode and photodetectors, couple the drive signal to the laser diode and to couple the outputs of the photodetectors to the measurement instrument.
U.S. Pat. No. 5,353,262 describes an ultrasound optical transducer that generates an optical signal the frequency of which varies in correspondence with acoustic energy incident on the transducer. The transducer includes a housing in which is disposed a signal laser. The signal laser is preferably a microchip laser, microcavity laser or the like. The signal laser has an optical cavity disposed between first and second reflectors and in which a lazing medium (also known as a gain crystal) is disposed. The reflectors are disposed on opposing plane-parallel surfaces of the lasing medium. An optical source injects an optical signal at a first frequency into the signal laser, which generates a second output signal at a second frequency. Acoustic energy impinging on the transducer causes the index of refraction of the optical cavity to change which in turn, causes the frequency of the signal laser to change. The frequency modulated optical signal from the signal laser is coupled to signal processing assembly that generates an output signal corresponding to the amplitude of the incident acoustic energy for use in imaging and analysis. An alternative embodiment is described where a piezoelectric device is positioned on the transducer for converting the acoustic energy into an electrical signal. The electrical signal is applied to electrodes on the signal laser. The electrical signal causes a change in the index of refraction of the optical cavity as a function of the acoustic energy applied to the piezoelectric device.
U.S. Pat. No. 4,196,396 describes the use of a Fabry-Perot enhanced electro-optic modulator to produce a bistable resonator that could be used as an optical switch, optical limiter, or optical memory device. A further embodiment taught by the ‘396 patent is an optical amplifier. The reference teaches the use of high voltage signals in the thousand voltage range to change the index of refraction of the electro-optic material in the Fabry-Perot cavity. Such a system does not lend itself for small signal probing applications.
U.S. Pat. No. 5,394,098 describes the use of longitudinal Pockels effect in an electro -optic sensor for in-circuit testing of hybrids and circuits assembled on circuit boards. In one embodiment, a layer of electro-optic material is disposed between opposing layers of optically reflective materials that include electrically conductive layers. The optically reflective layer having highest reflectivity to an applied optical signal is placed in contact with a conductor on the circuit board. The other optically reflective layer is coupled to electrical ground. An optical signal from a laser is applied orthogonal to the optically reflective layers on the electro-optic material. An electrical signal on the conductor of the circuit board produces a voltage potential difference across the optically reflective layers which varies the refractive index of the electro-optic material. A drawback to this design is that the orientation of the polarized optical signal is orthogonal to the orientation of the electro-magnetic field producing the Pockels effect in the electro-optic material. This reduces the sensitivity of the measured electrical signal. Further, forming electrically conductive layers on the opposing sides of the electro-optic material produces capacitive and inductive effects in the electro-optic sensor that limits the useful bandwidth of the system.
What is needed is a variable attenuation signal acquisition probing system using an electro-optical cavity that improves the sensitivity of the electro-optical cavity to applied electrical signals. Further, there is needed a voltage measurement system using a variable attenuation signal acquisition probing system with an electro-optical cavity with improved sensitivity.